Modelling and Simulation in High-Fidelity Crash Analysis of NGCTR-TD Composite Wing

Abstract

This paper presents a methodology that involves the development of high-fidelity modeling and simulation procedures aimed at supporting virtual certification for crashworthiness requirements specific to tiltrotor aircraft, addressing the critical need for accurate safety requirement fulfillment predictions and weight containment of wing. The unique crashworthiness requirement for tiltrotor wings necessitates a design that can ensure a controlled failure during survivable crash events. This is to alleviate the inertial load acting on the fuselage, thereby protecting occupants from injuries and fire while ensuring the integrity of escape paths. The objective of this methodology is to simulate the crash effects on the entire wing using explicit, non-linear, and time-dependent FE analysis. This approach verifies the spanwise placement of the frangible sections, the mode of failure, the loads acting on the fuselage links, and the acceleration transmitted to the structure. This study focuses on a standalone analysis.

1. Introduction

The T-WING consortium’s endeavor in designing and manufacturing the wing of the Leonardo Next Generation Civil Tiltrotor Technology Demonstrator (NGCTR-TD, Figure 1) represents a significant leap in aerospace engineering, particularly in the realm of tiltrotor technology.

This research project, as part of the Clean Aviation Fast Rotorcraft activities, delves into the development of high-fidelity modeling and simulation procedures to support virtual certification methods for tiltrotors’ crashworthiness requirements [2,3,4].

We have explored the intricate balance between aeroelastic stability and structural integrity essential for efficient high-speed forward flight. The use of advanced composite materials is pivotal in achieving this balance, offering both strength and flexibility while minimizing weight [5,6]. The methodology proposed encompasses Finite Element Analysis (FEA) of an aircraft drop test, a complex and detailed process aimed at simulating the structural behavior during an impact or drop event [7,8,9,10]. This analysis is critical for assessing the safety of an aircraft in emergency landing situations.

The NGCTR-TD project’s wing (Figure 2) is uniquely designed to ensure the integrity of the fuselage and the safety of passengers under the inertial loads transmitted by the wing mass in the event of a moderate, potentially survivable crash [11,12].

A notable design feature is the controlled failure of the wing during a crash to prevent fuselage collapse, as demonstrated in military applications like the Bell V-22 [13]. This concept of frangible sections near the wing–fuselage intersection is a pivotal aspect of the design, ensuring that in the event of a crash, these sections would break in a controlled manner, thereby alleviating the inertial load acting on the fuselage.

In vertical take-off and landing (VTOL) aircraft design, crashworthiness or impact resistance, crucial in conventional aircraft, becomes even more complex. This is due to the unique dynamics of VTOL aircraft, including hovering maneuvers that can lead to more significant impact damages compared to conventional aircraft. Additionally, the current absence of specific airworthiness standards for VTOLs, which instead adapt helicopter rules, adds to this complexity. FAA and NASA studies [14,15] have laid down principles for potential certification standards focused on minimizing passenger injury both before and after a crash, tying into the concept of survivability. This concept involves managing impact forces, ensuring the integrity of the fuselage’s occupant volume, and maintaining clear escape paths. Human tolerance to high-intensity impact accelerations, often greater than 10 g, is a crucial consideration for predicting the nonfatality of a crash. The design of the NGCTR-TD’s wing plays a significant role in ensuring the fuselage’s integrity and passenger safety under inertial loads in a survivable crash, incorporating controlled failure mechanisms. The study of the propagation of internal forces and inertial forces due to ground impact is key in enhancing the aircraft’s overall safety and survivability.

Recent advancements in aerospace composites emphasize the crucial role of modeling and simulation (M&S) in the design and development of aeronautical structures [16]. With the growing use of composite materials in aerospace, especially in aeronautics, M&S methods have evolved to support more reliable design practices and aid in meeting the stringent qualification requirements set by regulatory authorities [17]. The exploration of M&S advancements has opened doors to high-fidelity approaches across various disciplines, including structural strength, aeroelasticity, internal noise, thermal effects, electromagnetic compatibility (EMC), crashworthiness, and energy storage [18]. These advanced M&S methods demonstrate an exceptional ability to simulate the complex behaviors of aerospace composite systems, significantly reducing the reliance on physical testing. This shift towards a more digital qualification and certification process is a response to the need for more efficient and accurate design methodologies in aerospace engineering [19]. Furthermore, the push for greener aircraft and innovative systems for space exploration introduces new technical challenges that M&S methods must address. This includes the integration of onboard cryogenic hydrogen tanks for more environmentally friendly aircraft and the development of new types of ceramic composites for thermal protection in spacecraft. These emerging requirements highlight the necessity for M&S methods to evolve and adapt, taking into account previously unconsidered technical issues.

A crucial aspect of this evolution in aerospace composites M&S is the development of a Credibility Assurance Framework (CAF). This framework supports risk-informed applications of advanced M&S methods, ensuring that these sophisticated techniques can be reliably applied in practical aerospace applications [15]. The CAF is instrumental in bridging the gap between theoretical models and real-world applications, ensuring that the simulated results are credible and applicable to actual aerospace scenarios [18].

In summary, the advancements in M&S for aerospace composites are not only enhancing the reliability and affordability of aircraft and spacecraft development but also shaping the future of aerospace technology. These methods play a pivotal role in advancing the capabilities of aerospace structures and systems, meeting the challenges of modern aeronautics and space exploration [20].

In detail to this relevant topic, the design and effects’ evaluation of frangible section on the whole tiltrotor safety requirement is deemed crucial to qualify for flight with respect to regulations. The use of high-fidelity modeling and simulation is novel for this type of topic due to the following points:

• in terms of general approach, as previous experience was based on low fidelity modeling and static analysis (i.e., AW609);
• exploitation of high-fidelity modelling and simulation for viability of certification by analysis.

2. Materials and Methods

2.1. Wing High-Fidelity Model

The FE model plays a crucial role in the NGCTR-TD project, particularly in understanding how the aircraft’s design can influence occupant survivability during a survivable crash event. Given the high wing configuration of the NGCTR-TD, it is imperative to assess how the inertial loads from the wing mass, including structures, internal systems, and nacelle masses, impact the cabin and its occupants. The model is strategically designed to initiate a controlled failure in specific sections of the semi-wings (both left and right) during a crash. This is to mitigate the forces transmitted to the fuselage and, consequently, protect the cabin occupants.

The comprehensive wing finite element model (FEM) includes various components like the wing itself, movable surfaces, nacelle primary structures, and attachments to the fuselage. These elements are modeled using three-dimensional elements, either shell or solid, to accurately reflect the physical properties of the wing. Additionally, the model takes into account the mass of internal systems, fuel, bladders, engines, transmissions, rotors, and other components housed within the wing, using lumped mass elements for simulation. The analysis considers the full fuel mass condition, amounting to 12.196 lbs., to ensure a realistic assessment of the wing’s performance under different load conditions.

Utilizing the LSDYNA MPP single precision R12.0 software on a Cluster HPE Pro-Liant DL560 Gen10 with 64 parallel processors, the analysis provides detailed insights into the wing’s behavior under crash conditions. Starting from the wing model, a rigid plate representing the fuselage’s lift beams attachments was created. The wing links to this plate via spherical joints, and during the drop test, this plate impacts a deformable honeycomb absorber, simulating the fuselage’s stiffness contribution.

Key steps in the pre-process include detailed modeling of the wing, wing–fuselage links, and the absorber compatible with LS-Dyna’s requirements. The model employs a mixed-element approach, using shell elements for composite parts and 3D elements like CHexa, CPenta, or CTetra [21] for honeycomb and metal parts. One-dimensional elements represent connecting elements, masses, and joints. The modeling of the wing in the NGCTR-TD project involves detailed simulation of various materials and components. All parts of the wing, including the metallic and composite components, were modeled to replicate their real-world counterparts. Metallic parts of the wing, depicted in Figure 3, were simulated using material card MATL24,

Table 1. Mechanical properties of fabric CFRP-CYTEC 977-2-42-3KT300D-8H-372-1524.

Elastic Moduli–CFRP Fabric
Longitudinal Elastic Modulus-E1 (psi)	811,000
Transversal Elastic Modulus-E2 (psi)	791,000
In Plane Poisson ratio-ν12 (/)	0.05
In Plane Shear Modulus-G12 (psi)	570,000
Allowable–CFRP Fabric
Longitudinal Tensile Strength–F1T (psi)	113,260
Longitudinal Compressive Strength-F1C (psi)	98,100
Transversal Tensile Strength–F2T (psi)	102,070
Transversal Compressive Strength–F2C (psi)	97,700
In Plane Shear Strength–S (psi)	13,233

), an elasto-plastic material suitable for representing isotropic materials. This material card allows for accurate modeling of the stress–strain behavior of these components.

Composite components of the wing, shown in Figure 4, were modeled using primarily 2D orthotropic material MATL58 (Table 1 and

Table 2. Mechanical properties of UD CFRP.

Stiffness Moduli–CFRP UD
Longitudinal Elastic Modulus-E1 (psi)	2,170,000
Transversal Elastic Modulus-E2 (psi)	1,230,000
In Plane Poisson ratio-ν12 (/)	0.3
In Plane Shear Modulus-G12 (psi)	660,000
Allowable–CFRP UD
Longitudinal Tensile Strength–F1T (psi)	306,000
Longitudinal Compressive Strength-F1C (psi)	194,000
Transversal Tensile Strength–F2T (psi)	6000
Transversal Compressive Strength–F2C (psi)	5100
In Plane Shear Strength e–S (psi)	13,500

), different from the isotropic materials used for metallic components.

Figure 5 illustrates the thickness distribution of the composite parts of the wing, with the contour scale normalized to the maximum thickness value, the number of 2D elements utilized in the modeling process is reported alongside. This material card, based on Matzenmiller’s damage mechanics model with four Hashin’s failure criteria, is crucial for accurately simulating the post-failure behavior of composite materials, a key factor in structural crash analysis. In this case study, Hashin’s failure criterion was used to predict the initiation and progression of damage for composite parts. The reason for this choice is that it is a separate-mode criterion that, unlike others (such as Tsai–Hill or Tsai–Wu), allows us to discriminate whether the damage occurs at the fiber or matrix level, and to which stress it is coupled (tensile, compressive, shear), in order to better understand the composite structure behavior.

Moveable surfaces of the wing were converted from Nastran’s GFEM version, as shown in Figure 6. The model includes detailed representations of these components to ensure accurate simulation of their behavior.

Moreover, there are several parts, like the nose rib, the leading edge, and the tip and root ribs, that are made of aluminum alloy Al7050-T7451 (

Table 3. Mechanical properties of Al7050-T7451.

Mechanical Properties
Young Modulus (ksi)	10,400
Poisson ratio-ν12 (/)	0.3
Tensile strength-ultimate (psi)	76,000
Yield stress (psi)	68,000
Elongation at failure (%)	11

).

The fasteners in the wing are modeled as discrete beam elements using material card MAT196, which allows for elastic and elastoplastic springs with damping. This modeling approach is essential for accurately representing the mechanical behavior of fasteners under various load conditions.

For the contacts during a crash event, a single surface contact was introduced between all parts of the fuselage, except for 1D and 0D elements. This approach prevents penetrations and enables interaction between components. Additionally, an ERODING contact was introduced to account for the deletion of failed elements and exposure of new 3D faces.

In summary, the wing’s model in the NGCTR-TD project is a comprehensive simulation of its various components, employing specific material cards and contact models to accurately replicate the real-world behavior of each element under different conditions. The model comprises over 1.8 million nodes and 2.7 million elements, reflecting the complexity and detail of the wing structure.

2.2. Drop Test Configuration

Leonardo provided wing drop test data and configuration as a reference point, despite some differences with the actual tested wing. The key boundary conditions for these tests included a free fall from 3 m, impacting a deformable honeycomb structure that emulates the stiffness of the fuselage (

Table 4. Mechanical properties of honeycomb in sandwich structure.

Stiffness Moduli-HC		Allowable-HC
E1 (psi)	1	Tensile 0° (psi)	0.02166
E2 (psi)	0	Compressive 0° (psi)	0.01926
ν12 (/)	0	Tensile 90° (psi)	0.01936
G1Z (psi)	6500	Compressive 90° (psi)	0.06219
G2Z (psi)	3400	Shear in plane (psi)	0.03482

). This setup was replicated in the numerical simulation of the drop test. The model accounted for various elements such as moveable surfaces, fuel, and nacelle masses to ensure a comprehensive and realistic simulation, mirroring the conditions experienced during actual crash scenarios (Figure 7 and Figure 8).

2.3. Characterization of Absorber in Numerical Models

The absorber numerical characterization in the NGCTR-TD project is a complex and detailed component of the overall simulation model. This model intricately simulates the mechanical characteristics of a honeycomb structure, a critical element in the wing crash test. The absorber is composed of four distinct honeycomb blocks, organized in pairs and separated by aluminum sheets (Figure 9 and Figure 10).

These blocks are simulated using one-dimensional spring and damper elements with an experimental force–displacement curve, a strategic choice aimed at optimizing computational efficiency. This approach simplifies the representation of the honeycomb’s complex behavior during impacts. The compression behavior of these springs is depicted in a detailed graph (Figure 11) showcasing the force–displacement characteristics crucial for understanding the honeycomb’s response under a vertical drop scenario. The springs’ behavior is based on specific research findings [22], integrated into the Finite Element Model (FEM). The analysis considers the total wing mass, focusing on its center of gravity, and an initial velocity matching the experimental test’s drop height. This method provides accurate predictions of the absorber’s crushing behavior, mean collapsing force, and mean acceleration, which are vital for evaluating the honeycomb’s effectiveness as an absorber in crash scenarios. Furthermore, the maximum crushing experienced by the absorber during the drop test impact is 2.5 inches.

3. Results

The wing drop test simulation has been a critical brick in evaluating the crashworthiness of VTOL. In this specific case, the Finite Element Model (FEM) incorporated the entire wing mass, centered at its gravity center, and considered an initial velocity corresponding to a drop height of 3 m, mimicking the conditions of the experimental test.

During the analysis, we observed a consistent energy balance: the initial kinetic energy is transformed into internal energy as the crash phenomenon progresses. As can be seen in the Figure 12, the decision was made to terminate the analysis before the complete dissipation of kinetic energy. This decision was made because the focus of this work is the identification of the frangible section of the wing, and not the complete crash phenomenon, which would be studied by including the entire structure.

3.1. FEA Validation

Figure 13 in the analysis illustrates the time-wise value of the acceleration recorded at the wing’s center of gravity. Notably, the mean acceleration value aligned closely with the experimental data, validating the simulation’s accuracy. This aspect of the simulation is crucial for predicting critical factors like the absorber’s crushing behavior, mean collapsing force, and mean acceleration experienced by the wing during the crash.

3.2. Identification of Wing’s Frangible Sections

A significant outcome of the simulation was identifying the wing’s frangible sections, the areas most susceptible to failure during impact. The detailed FEM revealed that the wing breaks outboard the wing–fuselage intersection, as shown in Figure 14.

A closer examination of the internal wing bay (Figure 15) indicated that the failure involved critical components such as stringers, spars, and the lower panel. This finding is vital for designing wings that can fail in a controlled manner, thereby reducing the impact forces transmitted to the fuselage.

3.3. Composite Parts Damage Analysis

Further analysis focused on the damage propagation in the composite parts of the wing. Figure 16 presents a top and bottom view of the damage, with the failure path indicated by the red-colored elements. This modelling, grounded in Hashin’s failure criteria for orthotropic materials, provides an in-depth understanding of how damage propagates through composite materials during a crash.

3.4. Simulation Output Data and Its Implications

Beyond identifying the frangible sections, the simulation also provided crucial data on acceleration and forces acting on the wing during the crash event. Figure 17 compares the wing’s global acceleration path with experimental data, offering a benchmark for the simulation’s accuracy.

Additionally, the time-dependent forces acting on the wing–fuselage links were analyzed (Figure 18). These force values are instrumental in assessing the fuselage’s strength under crash loads, a key consideration in ensuring passenger safety and structural integrity. The time when the catastrophic failure of the lower panel begins was highlighted in the Figure 18. It is remarkable that almost all the measured forces in the fuselage attachments exhibit significant peaks at that instant. This observation provides additional knowledge about the structural behavior during critical phases, enriching the understanding of failure dynamics.

4. Discussion

The application of high-fidelity modeling and simulation (M&S) in aerospace engineering has seen remarkable advancements, particularly in the analysis of composite wing structures. This study delves into the high-fidelity analysis of an experimental wing drop test, utilizing state-of-the-art simulation techniques and materials modeling to understand and predict the behavior of aerospace structures under crash conditions. An important aspect of advancing M&S methods is the development of a Credibility Assurance Framework (CAF). This framework ensures that the advanced M&S methods are applied in a risk-informed manner, bridging the gap between theoretical models and real-world applications. The CAF is crucial for validating the simulated results and ensuring their applicability in actual aerospace scenarios.

The high-fidelity analysis of the wing drop test, particularly for complex aircraft such as VTOLs/tiltrotors, represents a significant advancement in aerospace engineering. The successful validation of the simulation results against experimental data underscores the reliability and sophistication of modern modelling techniques. This validation is not just a technical achievement; it has profound implications for the design and certification of special aircraft like VTOLs and tiltrotors.

The identification of the wing’s frangible section through this advanced modelling is a breakthrough, especially for tiltrotor aircraft. Tiltrotors, which combine the vertical lift capability of helicopters with the speed and range of fixed-wing aircraft, present unique engineering challenges. The wing’s integrity and its controlled failure in a survivable crash are critical to the safety and viability of these aircraft. The high-fidelity simulation accurately predicting the frangible section’s location and behavior under crash conditions is a testament to the advances in aerospace simulation technologies. This achievement is not just a step forward in aircraft design; it is a leap towards enhancing the safety of next-generation aircraft. For tiltrotors, this means the ability to design wings that can withstand operative loads while protecting the fuselage and passengers during crash. The simulation’s success in mirroring first experimental results gives engineers and designers a powerful tool to test and refine these critical aspects of aircraft design in a virtual environment. This capability is invaluable, as it allows for rigorous testing and optimization without the high costs and risks associated with physical prototypes.

Moreover, the credibility of these simulation methods, as evidenced by their alignment with experimental data, paves the way for their broader acceptance in the aerospace industry. This acceptance is crucial for the adoption of advanced modelling techniques in the certification process, particularly for innovative aircraft designs where traditional testing methods may be inadequate for cost and time reasons. The ability to accurately simulate and analyze complex behaviors of aerospace structures opens new possibilities for designing safer, more efficient, and more capable VTOL and tiltrotor aircraft.

Moreover, this advancement sets the stage for further studies that are already underway to create models and simulations of an entire aircraft, including the wing, fuselage, landing gear, and tail planes. These comprehensive models will undergo validation through a full drop test to verify the frangible sections [23]. Such holistic simulations are crucial in assessing the overall structural integrity and crashworthiness of the aircraft, providing deeper insights into how different components interact and respond during impact.

5. Conclusions

The strong points of this study are recognized in the stand-alone analysis of the wing with a focus on the frangible section path and on the preliminary validation of the methodology with respect to similar physical test. Once a wing test article of NGCTR is available, a full validation will be performed.

In conclusion, the successful application and validation of high-fidelity modelling in identifying the frangible section of a wing significantly impacts the aeronautical world, especially for VTOLs/tiltrotors. This advancement not only enhances the design and safety of these specialized aircraft but also signifies a paradigm shift in how aerospace engineering approaches the challenges of designing and certifying next-generation aircraft. It underscores the growing importance of sophisticated simulation methods in advancing aerospace technology, ultimately contributing to safer and more reliable air travel.